The actions of opiates and opioid peptides on opiate receptors located on neurons of the nigrostriatal dopamine system appear to increase dopaminergic activity by either enhancing release or acting in a synergistic manner with dopamine on post-synaptic sites. The cataleptic actions of opiates cannot, therefore, be due to blockade of dopamine transmission. Opiates act both in the periaqueductal gray matter and the nucleus gigantocellularis to produce analgesia. One effect of opiates at these brain levels is to activate descending inhibitory spinal pathways. These pathways appear to be coded, at least in part, by norepinephrine and serotonin. A number of peptides were found to be analgesic following microinjections into the periaqueductal gray matter. Among these were bombesin, neurotensin and bradykinin. Stress-induced analgesia was found to be partially antagonized by hypophysectomy and depletion of catecholamines.